assets/shell-integration: improve the integration with ble.sh

[akinomyoga]

Here are fixes for the shell integration. These are additional adjustments to 56e9a9a and 78ea214. Let me open this as a draft PR. There can be even more fixes because the check of the shell integration with ble.sh is still ongoing (Cc: @SuperSandro2000).

• 608aad4 In case that the shell option set -u is set, the optional variables are referenced as ${varname-}
• 9df1b53 The precmd/preexec hooks are registered only when it is not yet registered. This avoids duplicate precmd/preexec handlers after wezterm.sh is sourced multiple times. Maybe we also want to modify the code for bash-preexec?
• e2c0816 By default, ble.sh assumes that the cursor position and the terminal contents are not changed by calling PRECMD/PROMPT_COMMAND. However, it seems the escape sequence sent from the wezterm precmd handler causes a newline when the cursor position is not at the beginning of the line. This breaks the internal layout calculation of ble.sh. To tell ble.sh that PRECMD can change the layout, we set a blesh option.

Update 2022-03-04: I'm currently waiting for rcaloras/bash-preexec#129 being processed.

[wez]

I'm going to close this for now: when you're ready to submit it, please rebase and reopen.

[akinomyoga]

OK! Sorry for keeping this open for a long time. I actually wanted to adjust it after rcaloras/bash-preexec#129 is settled, but it seems to take some time for it to receive another review from some other contributor of bash-preexec. I'll reopen it after rcaloras/bash-preexec#129 is finally merged or closed.